Rechargeable batteries are commonly used in a wide variety of consumer, industrial and business products. Common examples of such products are video cameras, cellular telephones, laptop computers, and portable drills.
Although a variety of rechargeable battery technologies have been developed, rechargeable nickel-cadmium (NiCd) batteries have several advantageous properties. Their principal advantage is that they are relatively compact and light in weight for the amount of charge they are capable of storing. They are also fairly inexpensive relative to some other high performance battery technologies. As a result of these advantageous properties, NiCd batteries have achieved widespread acceptance for such products as cellular telephones and video cameras.
Although NiCd batteries have several highly advantageous properties, they are not without certain problems. These problems are basically twofold. First, NiCd batteries may be seriously damaged by overcharging. Thus, NiCd batteries must be charged with great care. Second, NiCd batteries exhibit a property known as "memory." If a NiCd battery is repetitively charged before it has been fully discharged, it gradually looses its full ability to store charge. Thus, for example, if a NiCd battery is repetitively charged after being only half discharged, it will eventually have only half of its original charge storage capacity. The memory problem can also drastically reduce the number of charge/discharge cycles that the battery can undergo during its service life.
The problems of overcharge and memory are well known. Attempts to avoid damaging NiCd batteries by overcharging them have followed several approaches. The simplest approach has been to activate the charger through a timer so that the battery charges for a predetermined period of time. However, this approach is generally ineffective because the discharge state of the battery at the start of the charging cycle is not known. As a result, timer controlled chargers usually either insufficiently charge or overcharge NiCd batteries. Also, the use of a timer does not prevent the battery from being charged from a state well above full discharge, and thus does nothing to solve the "memory" problem described above.
A temperature sensor is also sometimes used to sense when NiCd battery is fully charged. When a NiCd battery reaches full charge, the chemical process that stores electricity ends and electrolysis begins. Heat is generated by an increase in the battery's internal pressure caused by gases generated during electrolysis. Unfortunately, thermal inertia delays the migration of heat to the heat sensor. By the time a temperature increase is detected, the battery's cells may already be damaged by excessive gas pressure.
Other chargers for NiCd batteries attempt to solve the problem of battery overcharging by sensing the battery voltage. When the battery voltage reach a predetermined value, the charger either switches to a trickle charge or charging is terminated. The principal disadvantage of this approach is the difficulty of determining and setting the voltage at which normal charging is to terminate, particularly if the charger is used with batteries having different battery voltages.
A more complex approach to the overcharging problem has been developed, known as the "negative slope" technique. A battery charger employing the negative slope technique is commercially available from Cadex Electronics Inc. of Burnaby, British Columbia, Canada. This approach examines the battery voltage history to determine when the battery is fully charged. It is well known that the voltage of a NiCd battery gradually increases as it is being charged until it is fully charged. If the battery is then overcharged, the battery voltage is gradually reduced. Negative slope chargers detect the peak of the battery voltage and then either switches the charger off or to a trickle charge mode.
Most NiCd batteries have not been very effective at ambient temperatures above 45.degree. C. However, a line of NiCd batteries having an extended temperature range has been developed recently. These extended range batteries have presented still another problem in charging NiCd batteries because their proper charging times vary with ambient temperature.
One technique for varying the charging time of NiCd batteries as a function of temperature is described in U.S. Pat. No. 4,609,861 to Inaniwa et al. The Inaniwa et al patent discloses a battery charger in which a battery is first charged at a constant current until a predetermined state of charge is achieved. The battery is then charged at a constant voltage for a period of time which may be a function of the voltage of the battery. Specifically, a heat-sensitive resistor senses the temperature of the battery being charged and generates a corresponding reference voltage. The reference voltage is then used to control the duration of a timer circuit which, in turn, controls the duration of the charge period.
Some negative slope battery chargers, such as the charger sold by Cadex, also addresses the "memory" problem by fully discharging the battery prior to starting the charging cycle.
Although negative slope battery chargers do solve the major problems associated with NiCd batteries, they do so in a relatively expensive manner. Such chargers must be relatively sophisticated to record the battery voltage and then detect the voltage peak. As a result of this sophistication, negative slope chargers are relatively expensive, thus drastically limiting the market for such chargers.
Another approach to solving the problem of "memory" in NiCd batteries is described in U.S. Pat. No. 4,342,954 to Griffith. The Griffith patent discloses a battery charger for NiCd batteries in which the battery is discharged to a predetermined voltage prior to starting a charging period. In theory, the NiCd battery is then in a known, relatively low state of discharge.
The approach disclosed in the Griffith patent goes a long way toward solving the problem of "memory" in NiCd batteries in most cases. However, there are times when the optimum state of discharge of a NiCd battery does not occur at a fixed voltage. In other words, the voltage of a NiCd battery when it is discharged to an optimum value may vary under certain conditions. Griffith's approach thus sometimes fails to place NiCd batteries in the optimum state of discharge prior to starting a charging cycle.